The invention relates to a method for radical cross-linking of organic polymers in the presence of cross-linking enhancers.
Radically cross-linked polymers have attained increasing technical importance in recent years. The comparatively low prices of the thermoplastic and elastomer polymers which are mostly available on a large technical scale and are readily processable, especially of polyolefins, is appealing. The cross-linking process can be carried out in a technically relatively simple and cost-effective manner, especially for elongated materials such as cable and wire insulation, tubing and housing. The thermal-mechanical spectrum of properties of cross-linked polymers is substantially improved which makes possible an expansion of the field of application to temperatures above the softening range of thermoplastic polymers.
In radical cross linking, radicals, which react with each other to form cross-linked, bridges, are generated in the polymers. The radicals can be generated either through the thermal decomposition of labile chemical compounds such as organic peroxides, which are mixed into the polymers prior to the cross-linking process, or by irradiation of the polymers with high-energy rays, especially with accelerated electrons. While the peroxidic cross-linking has been practiced technically in the form of various processes for a long time, the technical advance of radiation cross-linking is still relatively recent and is still under development, especially in the cross-linking of thick-walled materials such as medium-voltage insulation of polyethylene.
The goal of all cross-linking processes is cost-effective, homogeneous cross-linking which is as fast as possible and is gentle on the material. For this purpose, so-called cross-linking enhancers are often added to polymers to be cross-linked. The known cross-linking enhancers increase the yield at cross-linking points or reduce the radiation dose or peroxide concentration required for attaining the required degree of cross-linking; however, the exact operation of cross-linking enhancers has not yet been determined.
It is important to use cross-linking enhancers, for instance, in the radiation cross-linking of polyvinylchloride (PVC) and in the pressure-less peroxidic cross-linking of polyethylene in a salt bath. Due to its radiation sensitivity PVC, which has a greater tendency toward polymer decomposition than toward cross-linking, cannot be cross-linked without a cross-linking enhancer. For pressure-less cross-linking in the salt bath, cross-linking enhancers accelerate the cross-linking process and prevent the formation of pores by gaseous decomposition products of the peroxide.
The use of cross-linking enhancers is also important in the radiation cross-linking of medium-voltage insulation or thick-walled tubes of polyethylene. During the radiation cross-linking hydrogen and heat are generated during the irradiation proportionally to the radiation dose, for example, with a dose of about 250 kGy required for sufficient cross-linking of stabilized polyethylene of low density (LDPE): a considerable quantity of gas; in the adiabatic case, a temperature increase in the polymer of about 100.degree. C. Since with the technically desired high production speeds, the heat cannot be removed sufficiently fast in the customary technology, the unavoidable result is therefore damage to the polymer material through pore formation. Using cross-linking enhancers, the dose required for cross-linking high-pressure polyethylenes can now be lowered to about 150 kGy or to even lower doses, whereby the development of heat as well as of gas can be reduced to non-damaging values.
In addition to the requirement of very high effectiveness, technically usable cross-linking enhancers must meet a multitude of further production-related requirements. Since they have to be mixed into the polymer and homogeneously distributed therein prior to the shaping, for instance, by extrusion, they should be highly polymer-compatible or polymer-soluble and should be thermally stable particularly for the shaping process which takes place at temperatures of up to 200.degree. C. In addition, the cross-linking enhancers should not evaporate during the shaping nor during the intermediate storage of the shaped material up to the cross-linking (off-line production), i.e., they must be as non-volatile as possible. In addition, the cross-linking enhancers should not have a negative effect on the properties which are important for the technical application of the polymer product. One cross-linking enhancer which is frequently employed on a technical scale is triallyl cyanurate (TAC). However, also other products such as dipropargyl maleate (DPM), dipropargyl succinate (DPS) or triallyl mellitate (TATM) are recommended for technical use (see, for instance, U.S. Pat. No. 4,113,595).
In spite of the large number of known cross-linking enhancers, no enhancers have as yet been developed which are highly effective and at the same time meet sufficiently the foregoing production-related requirements. Of the enhancers investigated most thoroughly including the acrylates and methacrylates, allyl compounds, propynyl compounds and maleinimides, while sufficiently effective as cross-linking enhancers, are relatively incompatible with polymers and are highly volatile. The maleinimides moreover are thermally unstable and cause incipient cross-linking of polyolefins at temperatures between 150.degree. and 200.degree. C. In the peroxidic cross-linking of polyolefins, losses of cross-linking enhancers can further occur due to insufficient polymer compatibility and excessive volatility. While these losses can be compensated by overdosing, the manufactured product thereby becomes more expensive.
It is an object of the invention, therefore to develop a process for radical cross-linking of organic polymers in the presence of cross-linking enhancers which meet the foregoing production-related requirements.